Alzheimer?s disease (AD) is an incurable neurodegenerative disorder in which neuroinflammation is increasingly recognized to play a critical function. While innate inflammation has been implicated in AD, little is known about the contribution of the adaptive immune response. Preliminary data featured in this application demonstrate a peripheral immune signature of AD characterized by increased numbers of highly differentiated CD8+ T effector memory CD45RA+ (TEMRA) cells. Strikingly, CD8+ TEMRA cells were also present in-patient cerebrospinal fluid (CSF) and T cell receptor (TCR) sequencing indicated their clonal expansion, suggesting antigen specificity of these adaptive immune cells. TCR cloning and peptide screens demonstrated specificity of a subset of clonally expanded AD CSF TCRs to the Epstein-Barr virus (EBV) BZLF1 antigen. These results provide the first evidence of clonal, antigen-experienced T cells patrolling the intrathecal space of brains affected by age-related neurodegeneration. This K99/R00 application will test the novel theory of a detrimental adaptive immune response contributing to AD pathobiology. In Specific Aim 1, AD blood-CSF T cell clonotypes will be related to CSF biomarkers. This approach will determine specific T cell populations in AD and whether these cells relate to disease severity. In Specific Aim 2, antigen identification screens will be used to detect the self/non-self-antigen(s) driving T cell clonal expansion in AD. These assays could uncover a novel therapeutic target or biomarker for AD. Specific Aim 3 will determine mechanisms of T cell-mediated neuronal death and resiliency in AD using induced neuronal (iN) cells co-cultured with patient CSF CD8+ T cells. These experiments will assess whether AD CSF CD8+ T cells mount cytotoxic effector responses to iN cells infected with EBV and/or to a molecular mimic of BZLF1. The candidate, Dr. David Gate, has extensive experience in T cell biology and has spent more than a decade studying AD. During the mentoring phase of this award, Dr. Gate aims to advance his knowledge in next-generation sequencing analysis, sophisticated statistical methods, antigen screening, iN culturing methods and CRISPR gene editing. Dr. Gate's mentor and co-mentor, Dr. Tony Wyss-Coray and Dr. Mark Davis, respectively, have comprehensive expertise in these areas. They will provide an enriching environment for Dr. Gate to develop as a prominent independent investigator in neuroimmunology research. As an independent investigator, Dr. Gate will leverage the training under this fellowship to comprehensively and quantitatively evaluate the interactions between T cell molecular components and neurodegeneration.